1. Field of the Invention
This invention relates to recording/reproducing apparatus and, more particularly, to novel and highly-effective recording/reproducing apparatus that employs magnetic heads and can detect clogging therof.
2. Description of the Prior Art
Clogging of the magnetic heads of a video tape recorder (VTR) badly affects the recording and reproducing operations of the VTR. This clogging of magnetic heads is caused by the deposition thereon, especially in the gaps thereof, of magnetic material coated on the base of the magnetic tape medium. Clogging of the heads is especially troublesome during recording.
To remedy this problem, the clogged state has been conventionally detected by a circuit such as the one shown in FIG. 1.
In FIG. 1, a rotary drum 41 is provided to which a pair of magnetic heads H.sub.A and H.sub.B having azimuth angles (a slant angle of the magnetic gap relative to the scanning direction) different from each other are attached with an angular distance of 180.degree. therebetween. A magnetic tape 42 is wrapped around the peripheral surface of the rotary drum 41 with a tape wrap angle of substantially 180.degree. and transported in the longitudinal direction of the tape. The rotary magnetic heads H.sub.A and H.sub.B are rotated through an angle of 360.degree. while the magnetic tape 42 is transported through a distance of one frame; that is, the magnetic heads H.sub.A and H.sub.B are rotated one revolution per one frame. The magnetic head H.sub.A scans the magnetic tape 42 in odd-numbered fields and the magnetic head H.sub.B scans the magnetic tape 42 in even-numbered fields.
Video signals reproduced by the magnetic heads H.sub.A and H.sub.B in a given reproducing period, for example during edit recording, are supplied through signal amplifiers 43A and 43B to terminals or contacts A and B of a head changeover switch 44. The head changeover switch 44 is further supplied with a so-called RF switching pulse SWP which is synchronized with the rotating phase of the magnetic heads H.sub.A and H.sub.B as a changeover control signal. A movable contact C of the head changeover switch 44 is connected to the terminal A in one field period during which the magnetic head H.sub.A scans the magnetic tape 42 and to the terminal B in one field period during which the magnetic head H.sub.B scans the magnetic tape 42.
Reproduced signals successively supplied as outputs from the changeover switch 44 are supplied to a peak detecting circuit 45, the detected output of which is supplied to a comparator 46 wherein it is compared with a reference voltage V.sub.REF. The output signal from the comparator 46 indicative of the comparison result assumes a high level or "1" when the detection ouput from the peak detecting circuit 45 represents a normal reproduced video signal. On the other hand, when clogging occurs in the magnetic heads H.sub.A and H.sub.B, signals are picked up barely or not at all from the recorded track on the magnetic tape 42, so that the detection output assumes a low level or "0" and consequently the output signal from the comparator 46 assumes the low level or "0".
The output signal from the comparator 46 is supplied to a display device 47 which indicates whether or not clogging has occurred.
The clogged state detecting circuit described above is disclosed, for example, in laid-open Japanese utility model publication No. 59-60724.
In the conventional clogged state detecting circuit shown in FIG. 1, the reference voltage V.sub.REF supplied to the comparator 46 should of course be set at a level higher than the noise level. If the normal level of the reproduced video signals is assumed to be 0 dB, the noise level is approximately -20 dB. Thus, the level of the reference voltage V.sub.REF should be set at about -14 to -15 dB. However, scattering resulting from thermal characteristics and so on makes it extremely difficult to adjust the level of the reference voltage V.sub.REF with the required accuracy. If the level of the reference voltage V.sub.REF is set at too small a value, then, because of noise, the clogged state cannot be detected. On the other hand, if the level of the reference voltage V.sub.REF is set at too large a value, even a small decrease in the level of the reproduced video signals is interpreted to indicate the clogged state. Thus, the clogged state detecting circuit of FIG. 1 is lacking in accuracy and reliability.